python: use gi.require_version() in generate-setting-docs.py and examples

gi now emits a warning when not loading a specific library
version [1]:

  ./generate-setting-docs.py:21: PyGIWarning: NM was imported without specifying a version first. Use gi.require_version(NM, 1.0) before import to ensure that the right version gets loaded.
    from gi.repository import NM, GObject

Seems require_version() is reasonably old to just always use it without
breaking on older versions [2].

[1] Related: https://bugzilla.gnome.org/show_bug.cgi?id=727379
[2] https://git.gnome.org/browse/pygobject/commit/?id=76758efb6579752237a0dc4d56cf9518de6c6e55

examples/python/gi/firewall-zone.py

@@ -20,6 +20,8 @@
 #
 
 import sys
+import gi
+gi.require_version('NM', '1.0')
 from gi.repository import GLib, NM
 
 #